During countdown activities, a low-level fuel cut-off sensor located inside the External Tank failed a routine prelaunch check. The sensor protects a Shuttle’s main engines by triggering their shut down in the event fuel runs unexpectedly low. The sensor is one of four inside the liquid hydrogen section of the External Tank.

The External Tank’s liquid oxygen and liquid hydrogen were drained this evening. While the tank was being emptied, engineers monitored and collected data on the liquid hydrogen sensor that failed. They will continue to collect and analyze data overnight.

Space Shuttle Program managers plan a series of meetings tomorrow to discuss the problem and determine the steps necessary to get back into the launch countdown.

The STS-114 crew will remain at Kennedy Space Center for now while engineers work on the problem.

During their 12-day Return to Flight mission to the International Space Station, Discovery’s seven crew members will test new techniques and equipment designed to make Space Shuttles safer. They’ll also deliver supplies and make repairs to the Space Station.